Cycloolefin random copolymers obtained by copolymerizing ethylene with specific cycloolefins are well balanced among optical qualities, mechanical properties and thermal properties, and they are used as optical materials such as optical memory disc and optical fiber.
The cycloolefin random copolymers referred to above have been heretofore prepared by copolymerizing ethylene with specific bulky cycloolefin in a hydrocarbon solvent such as toluene, cyclohexane, hexane and heptane or using the cycloolefin itself as a solvent in the presence of a catalyst formed from a soluble vanadium compound and an organoaluminum compound.
For obtaining such copolymers as mentioned above in a high yield, a concentration of the copolymer in the polymerization system is increased, and a viscosity of the polymerization system is also increased, thereby to bring about various problems such that polymerization reaction heat is hardly removed, monomer composition in the system is hardly kept in the uniform state and transportability of a polymerization liquid is reduced. Especially when a copolymer of high molecular weight is intended to be prepared, the viscosity of the polymerization system becomes too high, and hence removal of polymerization reaction heat and keeping of uniformity of the monomer composition become much more difficult. For these reasons, a copolymer of uniform composition, particularly a copolymer of high molecular weight, is hardly prepared in a high yield in the polymerization system of high concentration by the use of conventional techniques.
As one example of methods to solve those problems, there is known a method of raising the polymerization temperature to decrease the viscosity of the polymerization system. However, in the case where a cycloolefin random copolymer is intended to be prepared using a catalyst formed from a soluble vanadium compound and an organoaluminum compound both conventionally used, this method still has such a problem that the catalyst activity generally lowers according to rising of the polymerization temperature.
Accordingly, there has been eagerly desired an advent of a process for preparing a cycloolefin random copolymer in which copolymerization of ethylene and specific bulky cycloolefin can be conducted with high catalyst activity in the polymerization system at a high temperature and at a high polymer concentration, and thereby a cycloolefin random copolymer of uniform composition and high molecular weight can be prepared in a high yield.
In the light of the above-mentioned conventional techniques, the present inventors have earnestly studied on a process for preparing a cycloolefin random copolymer. As a result, they have found that a cycloolefin random copolymer can be obtained in a high yield by a process of copolymerizing ethylene and specific bulky cycloolefin in the presence of a specific catalyst and that the catalyst activity has small temperature dependence, and accomplished the present invention.